The reason is, when I read a historian's work, I am thinking in terms of 'what is the method, what is the evidence base, how do they handle topics I know well' and for the non-expert reader, you'd have to learn the historical method and the evidence in detail and that's just...building expertise.
The point at which you can read work by a historian and critically assess its sources, method, theory foundation and level of care with details, you have at that point walked the 'road of training' at least in that subfield. But that road is not short!
I am a physicist, and the steps you've described are exactly what I'd have to go through if I wanted to understand work outside my sub-field too. So, that's very fair. Thanks for answering.